Valid Palindrome


Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,

"A man, a plan, a canal: Panama" is a palindrome. "race a car" is not a palindrome.

Note: Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.



// Valid Palindrome
// 时间复杂度O(n),空间复杂度O(1)
class Solution {
    public boolean isPalindrome(String s) {
        s = s.toLowerCase();
        int left = 0;
        int right = s.length() - 1;
        while (left < right) {
            if (!Character.isLetterOrDigit(s.charAt(left))) ++left;
            else if (!Character.isLetterOrDigit(s.charAt(right))) --right;
            else if (s.charAt(left) != s.charAt(right)) return false;
            else { ++left; --right; }
        return true;


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